A variety of products are manufactured as continuous sheets of material. For instance, products produced in sheet form are common in the paper, ceramic, aluminum, steel and glass manufacturing industries. It is often necessary to monitor sheet thickness during production as a quality control measure. For years, several industries have publicized the need for improved systems capable of more accurately and efficiently measuring the thickness of rapidly moving sheet materials. A number of non-contact measuring systems have been devised for performing such measurements.
Mass gauges are the most commonly used devices for performing thickness measurements of moving sheet material. Mass gauge technology is based on gamma- or x-ray radiation attenuation, whereby a radiation source and a detector are positioned on opposing sides of the moving sheet. The radiation source emits photons which pass through the sheet. The quantity of photons passing through the sheet per unit time is measured by the detector. Use of this technology is limited due to its high nonlinear dependence on the elemental composition of the sheet material. For this technology to be accurate, the precise alloy composition of the measured sheet must be known. The usefulness of radiation attenuation gauges is further restricted by the limited strength of available radiation sources. In other words, known radiation sources can only emit a limited quantity of photons in a given time. As a result, thickness calculations require time-averaging of measurements, precluding the collection of prompt measurements. As sheet movement speed is increased, the quantity of photons detected per unit sheet length decreases, thereby limiting the quantity of photons available to measure thickness and resulting in an increase in measurement uncertainty.
Optically-based thickness gauges provide an alternate means for measuring sheet thickness. Auto-focus profilometry is one example of an optically-based thickness gauge technology. Here, detectors operate by focusing a light beam onto a surface through a movable lens. Reflected light traveling along a uniform path is deflected by a beam splitter and then directed toward a pair of photodetectors. The photodetectors are arranged such that each detector receives an equal portion of light when the measurement surface is located at the focal point of the movable lens. As the measured surface is moved, the ratio of light intensity incident on the respective detectors shifts. The ratio of electrical signals transmitted from the photodetectors is used to reposition the movable lens such that the surface remains in focus. Knowledge of the focal characteristics and lens positions are used to calculate the distance to the surface. A more detailed description of this technique is found in U.S. Pat. No. 5,696,589. The accuracy and measurement speed of this technique is limited due to its incorporation of a mechanically moving component. Moreover, commercially-available systems often accept only a limited range of object motion, do not provide a means for correcting for films disposed on the surface being measured, and require complex and expensive components.
Optical triangulation-based profilometry is another example of a known optically-based measurement technology. The general concept underlying triangulation-based distance measurement is as follows: as an observed object moves relative to a fixed illumination and observer system, the object's movement results in a predictable change in its observed position. Generally, a light beam is focused on the surface of the object being measured and light scattered from the object surface is reflected at a known angle (.alpha.), through an imaging lens, to form a spot upon a position-sensitive photodetector. The relative location of the spot on the photodetector is defined as the spot centroid position. Referring to FIG. 1, changes in the distance between the optical system and the surface being measured result in a corresponding change in the position of the spot on the photodetector. Here, the dotted lines represent a displacement of the measured surface and the corresponding displacement of the reflected light beam. The relationship between surface displacement, t, and other optical system variables is defined by the following formula: EQU t=[t'd]/[(b sin.sup.2 .alpha.)+(t' sin .alpha. cos .alpha.)]
where,
t'=the displacement of the spot on the photodetector PA1 d=the distance between the incident light beam and the intersection of the reflected light beam with the imaging lens PA1 b=the distance between the imaging lens and the photodetector PA1 .alpha.=the angle of reflection of the incident light beam
With knowledge of the distance between the respective optical systems, triangulation measurements performed on opposing surfaces of a sheet material can be used to calculate sheet thickness. Sheet thickness is calculated by subtracting the distance between the optical systems and the respective upper and lower sheet surfaces from the distance between the respective optical systems. However, dual-side triangulation is generally not used due to the relatively slow speeds at which available optical measurement devices operate. To incorporate a triangulation-based measurement system for calculating moving sheet thickness, it is necessary to provide a measurement device capable of accurately performing measurements at a much higher rate than is possible using available optical measurement systems.
For the foregoing reasons, it would be desirable to provide an optical measurement apparatus useful for performing simultaneous high-speed triangulation measurements on opposing sides of a moving sheet of material. It would be further desirable for the aforementioned apparatus to be amenable to performing high-speed thickness measurements for sheet materials having translucent surface coatings and/or highly-reflective surfaces.